Multi-layer structures are used quite frequently in microwave, rf and other high frequency applications. The structures have a great deal of advantage because of their ability to reduce the overall area of circuit board structure which has clear benefits in the wireless industry where size considerations are of the utmost importance. Additionally, multi-layer boards have been accepted in the industry because of relatively low cost and good performance. However, isolation of neighboring transmission lines remains a great problem in the high frequency multi-layer structure.
A typical multilayer board structure in cross-section is shown in FIG. 1. In this particular structure, a lower and upper ground plane 101 enclose selectively located signal lines 102, 103 and ground layers 104, 105 and 106. This structure has improved isolation characteristics which are desired provided the ground layers 104, 105 and 106 are maintained at a good ground level. Clearly, the isolation between the rf signal lines can be reduced significantly compared to when the ground lines 104, 105 and 106 are floating. To this end, the reduction in isolation can be as great as 65 dB relative to the case when the grounds float. The lines 104, 105, and 106 are therefore connected to ground using metal coated via holes. One approach suggests placement of numerous vias that are small in diameter and are placed as close as possible. Another approach suggests placement of vias with separation less than a quarter wavelength at the frequency of interest. However, this is insufficient to provide adequate isolation and is sometimes counter productive. Additionally the importance of via inductance to isolation degradation is not understood.
Additionally, it is often necessary to have signal line transitions between layers of the multi-layer board which can adversely affect the isolation between the signal lines. For example, a typical cross-sectional view of a signal line in a transition between levels is shown in FIG. 2. The uppermost and lowermost ground planes for the transition in FIG. 2 are shown as 202 and 201. Additionally, an intermediate ground plane within the board for purposes of isolation is shown in FIG. 2 as 203 and 204. The signal line 205 makes a transition from one level to another for various and sundry purposes. This transition can be problematic from the standpoint of isolation. In this transition, a neighboring signal line to 205 could readily couple to signal line 205 if the grounding transition between the layers 201, 202, 203 and 204 proves inadequate. Accordingly, it is desired to have a better ground capability at levels 203, 204 in order to effect good isolation between signal line 205 and a neighboring signal line not shown. To be clear, the discontinuities at transitions result in the creation of other modes at the transition which can couple to neighboring signal lines.
As the overall performance of the system is hindered by poor isolation of signals at a transition, it is imperative to have a board which has improved isolation. While vias are used for grounding purposes, there is a need to effect proper grounding. Accordingly, what is desired is an improved isolation scheme in multi-layer rf and other high frequency transmission line circuit boards in which the isolation is maintained at an acceptable level.